principles of tonometry Flashcards
1
Q
why is a positive intraocular pressure required
A
to maintain the shape of the eye
2
Q
what balance is pressure largely determined by
A
aqueous production & outflow (drainage)
3
Q
what is the mean IOP
A
15.7mmHg (approx 16mmHg)
4
Q
what is the (one) standard deviation of 16mmHg IOP
A
2.5mmHg
5
Q
what is the mean IOP (16mmHg) plus 2x the standard deviation (2.5mmHg)
A
21mmHg (± 5 mmHg)
6
Q
how many % of the population fall into the category of ± 2 SD of 16mmHg & IOP less than 22mmHg
A
95%
7
Q
what should be done if IOP of 22mmHg or above is fun with non contact tonometry
A
check with goldmann application tonometry
8
Q
what is the goldmann application tonometry considered as
A
the gold standard of tonometry
9
Q
list the 5 long term/demographic effects on IOP
A
- age
- gender
- genetically determined
- myopia
- race
10
Q
how does age have an effect on IOP
A
IOP rises by 1-2mmHg between 20-70 years old
11
Q
why does IOP increase by 1-2mmHg from the age of 20-70
A
due to systolic blood pressure increase as we age
12
Q
how does gender affect IOP
A
IOP is 1-2mmHg higher in older women
13
Q
how does genetics affect IOP
A
family history can indicate low or high IOP
14
Q
how does myopia affect IOP
A
associated with higher IOP
15
Q
how does race affect IOP
A
IOP is higher in non glaucomatous black population than caucasian population
16
Q
what causes the IOP to pulsate
A
the cardiac cycle, as the choroid fills with blood and is emptied i.e. it fluctuates
17
Q
what is another name for the way in which time of day affects IOP
A
diurnal range (varies throughout the day)
18
Q
what is a normal diurnal range considered to be throughout the day
A
3-5mmHg
19
Q
what is considered to be a glaucomatous diurnal range
A
13mmHg
20
Q
what diurnal range is considered to be pathological (assess for glaucoma)
A
10mmHg
21
Q
what are the two groups in regards to peak IOPs throughout the day
A
1) higher in mornings with afternoon dip especially in males
2) some people have afternoon peaks (varies throughout the day)
22
Q
what should be done in order to monitor someones diurnal IOP range
A
repeat IOP measurements at different times of the day
23
Q
what must always be recorded when measuring IOP, especially when monitoring the diurnal range
A
the time of the day
24
Q
how does drinking water affect IOP
A
increases IOP by up to 3mmHg, with maximum effects at 20 minutes
25
how does alcohol affect IOP
decreases IOP by up to 3mmHg, with maximum effects at 5 minutes
26
how does coffee affect IOP
increases IOP by up to 3mmHg, with maximum effects at 20 minutes
27
how do you carry out a provocative test for glaucoma
drink one litre of water and measure IOP after 15 minutes and/or lying down in the dark for 1 hour
a rise in IOP of 8mmHg suggests a risk of glaucoma due to decreased aqueous outflow
28
list reasons why it is important to measure IOPs
- raised IOP is one of the major risk factors of glaucoma
- glaucoma causes optic nerve fibres to atrophy/die
eg, optic neuropathy, visual field defects & possible blindness
- reducing IOP can slow down the progression of optic neuropathy
- early detection is beneficial
29
list the 4 different types of IOP measuring techniques
- manometry
- indentation tonometry
- applanation tonometry
- non contact methods
30
how is manometry carried out
a probe/pressure sensor is inserted into eye and pressure is measured
(not popular with patients)
31
in indentation tonometry, what is the amount of indentation proportional to
the pressure inside the eye
32
what other technique is indentation tonometry less accurate than
applanation tonometry (but gives an estimate of IOP)
33
what is indicated when the cornea is more indented in indentation tonometry
low pressure
34
what is indicated when the cornea is not indented much in indentation tonometry
high pressure
35
what is the tool in indentation tonometry which creates the indentation
a weight
36
what does the weight in indentation tonometry determine
the pressure of the eye & is proportional to the pressure in the eye
37
what is the name of an indentation tonometer
schiotz tonometer
38
where in the eye does the probe of the indentation tonometer go
into the cornea
39
what is the probe attached to with the indentation tonometer
a scale where you can add variable amounts of weight
40
what weight should you start off with in indentation tonometry
5.5g of weight onto the probe and increase if no indentation due to high pressure
41
what must be taken into account which resists the indentation during indentation tonometry
the rigidity of the globe
42
what is supplied with the indentation tonometry instrument
conversion tables - translates the scale readings into an estimate of the IOP
43
what is the calculation of the pressure with indentation tonometry (e.g. when using the schiotz tonometer)
Pt = F/a + b.s
Pt = pressure
F = force applied
a & b = constants
s = scale (amount of indentation occurred)
44
why is the calculation for indentation tonometry not accurate
b & a vary between people as everyone has different scleras i.e. variance between eyes
45
where is indentation tonometry still used
vets
46
what is the name of the dynamic or rebound tonometer
icare tonometer
47
what is not required when using the icare tonometer
anaesthetic
48
how does the icare tonometer work
a magnetised probe is launched towards the cornea and bounces back
very rapid, px won't feel the probe hit there cornea
49
what does the probe of the icare tonometer consist of
a magnetised steel wire shaft covered with a round plastic tip at the end
50
what occurs with the icare tonometer with someone with a high IOP
probe will decelerate more rapidly
51
what occurs with the icare tonometer with someone with a low IOP
probe will decelerate less rapidly
52
what do patients prefer icare dynamic or rebound tonometry over and why
non contact tonometry, as its less invasive
53
what does the tonopen require
anaesthesia
54
what two things does the tonopen involve, which makes it difficult to classify
both applanation and indentation processes
55
how does the tonopen work to measure IOP
it has an applanation service that flattens the cornea & a tiny plunger protruding microscopically in the centre, the transducer senses the indentation produced by the probe
56
how is the IOP measurements displayed on the tonopen
digital readout
57
how large is the tonopen
it is a small handheld battery device
58
what are the 3 advantages of the tonopen
its portable, accurate and easy to use
59
list all 3 instruments used for indentation tonometry
- schiotz tonometer
- iCare tonometer: dynamic or rebound tonometry
- Tonopen: involved applanation and indentation
60
state the theory behind the imbert-fick principle
for a spherical container which has an infinitely thin, perfectly elastic and dry surface, equilibrium is achieved for an object placed in contact when:
Pt = F/A
```
Pt = pressure (within the spherical object)
F = force applied
A = area of applanation
```
61
state 3 reasons why the cornea/eye does not fulfil the required criteria of the imbert-fick principle and state the modified imbert-fick calculation that applies to a surface like the cornea
- the cornea is not infinitely thin
- the cornea is not perfectly elastic
- the corneal surface is moistened by the tear film
Pt = F/A - k + t
```
Pt = pressure
F = force applied
A = area of applanation
k = rigidity of eyeball (which pushes tonometer AWAY)
t = surface tension (force to tears which pushes tonometer TOWARDS the eye)
```
62
what causes the surface tension of the tears to pull the tonometer towards the eye
the tear film exerts a capillary pressure
63
which two components of the eye exert their own pressure when doing applanation tonometry
- force of the cornea k (pushes away)
| - tear film/surface tension t (pulls towards)
64
when are k and t of the imbert-fick principle cancelled out (i.e. approx. equal) and state the new calculation
when the diameter of the circular area of applanation is between 3-4 mm
then rigidity of eyeball and surface tension are approx. equal to one another and cancel each other out
Pt = F/A
```
Pt = pressure
F = force applied
A = area of applanation
```
65
state method A of applanation contact tonometry
- apply a known force
| - measure area applanated
66
state method B of applanation tonometry
- increase force gradually until...
| - known area is applanated
67
how much is the circular area of applanation with the goldmann tonometer, and what does this mean
7.35mm2
diamater = 3.06m
this means the imbert-fick principle applies, as the diameter is between 3-4mm, so we can assume that surface tension of fluid and rigidity of the cornea cancel put
68
what is the advantage to the fact that the area of applanation is 3.06mm in goldmann tonometry
1g = 10mmHg (if the diameter is 3.06mm)
| so no conversion tables are needed
69
what does 2g of weight of the goldmann tonometer = in IOP
20mmHg (just x by 10)
70
what does the observer look through when using the goldmann tonometer
the tonometer head
71
when does the circular flattened tonometer head of the goldmann tonometer become larger, and when do you know that you have full applanation
when you increase the force on the area applanated
| full applanation is when the diameter of the circular flattened area = 3.06mm
72
what does the bi prism tonometer head do in goldmann tonometry
it splits the fluorescein ring into two by 3.06mm (a top half and bottom half) when the tonometer head is correctly centred on the cornea
73
what happens to the two semicircular images with the bi prism goldmann tonometer when force is applied to the cornea
they two semicircular images get closer together
74
when do you know you have achieved full applanation with the bi prism goldmann tonometer
when the inside edges of the tow semicircles are just touching
75
list the full method of how to carry out goldmann applanation tonometry in practice
- use a disposable probe
- position the patient on the chin rest of the slit lamp
- anaesthetise the cornea
- apply flourescein using a flourescein paper strip
- set 1.0gm or 1.5gm force
- insert cobalt blue filter in slit lamp, so that the flourescein glows
- bring probe into contact with the cornea by moving the slit lamp forward (looking from each side and above to keep the probe entered onto the cornea)
- when correctly centred on cornea, the semicircle will be of equal size to each other
76
what must you do if you observe one semicircle to be larger/unequal to the other semicircle i.e. not centred on the cornea
move the tonohead/probe towards the larger semicircle, to equalise them WITHOUT touching the eye/cornea
77
what is the hand held version of the goldmann tonometer called
perkins tonometer
78
what are the advantages to the perkins tonometer
- can use on a wheelchair px
| - domiciliary visits
79
what theory does the non-contact tonometer share and state one difference
same as applanation tonometry theory but puff of air is applanated force
80
what are the two different ways to measure IOP with the non-contact tonometer
air puff is gradually increasing force applied, either measure:
- time to achieve applanation
or
- force of air used when applanation achieved
81
list 3 advantages of non-contact tonometers
- no 'physical' contact with cornea = less risk of corneal abrasion
- no anaesthetic required
- can be performed by non optometric staff
82
state a disadvantage to the non-contact tonometer
can over estimate high or low pressures
83
what are the three main components used to measure IOP in a non-contact tonometer and how does it measure the IOP
- IR emitter
- Air puff nozzle
- IR detector
IR light is bounced off the cornea and picked up by the IR detector by the other side of the cornea, but a spherical cornea will bounce some IR away and it won't reach the IR detector, so thats why the air force is applied to the cornea by the air puff nozzle, so an area of the cornea is applanated/flattened. the flattened bit of the cornea emits a greater amount of IR light into the detector, so the tonometer knows the applanation has occurred
84
which type of measure of IOP does the Reichart non-contact tonometer use
time taken to flatten the cornea (so IR signal picked up by the detector increases) measured
it is calibrated against the goldmann to give output in mmHg
85
what is the name of the handheld non-contact tonometer and which type of measurement of IOP does it use
keeler pulsair
uses transducer to sense the air pressure at the moment of applanation (i.e. force of air used when applanation achieved)
86
why are multiple readings required when using a non-contact tonometer
at least 3-4 readings should be taken due to cardiac cycles as it has peaks and troughs, so needs to be averaged and time should be noted
87
what two things is IOP measurements influenced by
- corneal thickness
| - corneal curvature
88
how does a steep cornea estimate IOP
over estimates IOP
89
how does a flat cornea estimate IOP
under estimates IOP
90
how does a thin central cornea estimate IOP
under estimates IOP
91
how does a thick central cornea estimate IOP
over estimates IOP
92
what is the average central corneal thickness and the range
approx. 0.54mm
| range: 0.44mm - 0.64mm
93
what instrument/measurement is used to measure corneal thickness in optometric practice
pachymetry
94
what must be instilled into px eye before carrying out pachymetry and contact tonometry
anaesthesia
95
what three things must you check on the anaesthetic, before using it on a px
- expiry date
- drug concentration
- batch number
96
how long does it take for the cornea to anaesthetise and how does it last for
1 min to anaesthetise and lasts for 25 mins
97
list the three different corneal anaesthetics that can be used
ester type:
- tetracaine (amethocaine) hydrochloride 0.5%
- oxybuprocaine hydrochloride (benoxinate) 0.4%
- proxymetacaine hydrochloride 0.5%
